This invention relates to method and apparatus for removing coherent noise from an electrical signal, such as powerline noise, with the noise cancelling taking place in real time. That is, the noise cancellation occurs at the time the echo signal is received, rather than being performed at a later time on previously recorded signal data.
Typical applications of the invention are in the field of noise cancellation in radar, sonar and seismic echos. In a typical system electromagnetic pulses of high energy are periodically transmitted from a source and in the interval between transmission pulses, echos are detected and recorded or displayed at a receiver, providing information on objects in the path of the transmitted pulses. The echo signals are often faint and the relevant information in the received echo is often masked by noise, including coherent noise from various sources such as powerlines, generators, and the like. One such pulse transmission and receiving system is described in U.S. Pat. No. 3,263,160, and one of the objects of the present invention is to provide a new and improved coherent noise cancelling filter and process for filtering suitable for removing or cancelling coherent noise in the received signals of the system such as is disclosed in said patent, as well as in other radar, sonar and seismic type of systems.
Coherent noise refers to a noise from an electrical source which has a fundamental frequency and harmonics, and the level of the noise as well as the frequency should be substantially constant over a relatively short period which depends upon the time constant of the filter, but typically is in the order of a few seconds.
The problem of coherent noise in an echo detecting system is not new. In a typical detecting system, the signal sensor responds both to the signal S and coherent noise N. A typical waveform obtained at the receiver is illustrated in FIG. 1. In operation, the attitude and/or the location of the sensor may be changed with each reading, which in turn varies S and N independently of each other. In geophysical exploration and the like, the detecting system is utilized in a variety of terrain, and light weight and ease of mobility are desirable characteristics. The party utilizing the equipment has no choice for placement and hence has to operate in areas where electromagnetic fields from powerlines, generators and the like produce considerable coherent noise.
The detecting system employs a time-domain principle. The transmitter is located at a fixed position and produces an electromagnetic field pulse of known rectangular form. The sensor responds to this field to give a signal St. The desired anomalous signal S occurs during a period after St. There is a period of time Tn before the next transmitter pulse when only the noise N is present. The anomalous signals are of a decaying form (an exponential is illustrated). The form of the reading must be recorded for interpretation and its interpretive value is related to the dynamic range of the recording. Noise restricts the lower dynamic range. The desirable dynamic range is of the order of 1000:1. It should be noted that St from the sensor varies with attitude and location of the sensor. The ratio of the received maximum transmitted signal St to the received maximum anomalous signal S' can be greater than 1000.
To economically define the signal for recording, quasi-logarithmic compression normally is used. At low levels, the response is linear. At high levels the response is logarithmic and known. This compression provides an approximated constant percentage resolution. Also, it assists in the time recovery of the high overload of the transmitted signal St. The amplifier should have little or no phase or time distortion over a band 0-10,000 Hz.
Similar noise problems exist for other systems employing electrical potential or electromagnetic sensors. These systems have practical acceptance but there is demand for their use in developed areas where these noise problems exist. It is desirable to have a modification or a separate addition for these systems to extend their utility. This approach can minimize equipment inventory and maintain standard operating procedures. It is an object of the present invention to provide method and apparatus suitable for use with such systems for cancelling the coherent noise.
Attempts have been made in the past to overcome this problem, but have not been satisfactory with modern day equipment. The older designs of geophysical equipment operate on a continuous signal or frequency domain principle at discrete frequencies. The noise in these systems can be reduced by choice of operating frequencies and use of accepted filter technology. The newer designs of instruments operate on an intermittent or time-domain principle. The transmitted signal and the desired received signals occur at different time periods. These systems are cost effective for broad spectrum analysis. The sensors and processing of the received signals require a broad frequency response for the spectrum of interest. It is not practical to remove the man-made noise with conventional band rejection filters. This filtering introduces discontinuities in the pass-band of the signal and degrades the results.
Computers have been used to filter signal records. Preferably this requires a continuous digital record of the signal at equal time intervals. Filtering in this manner requires a high degree of absolute accuracy in the digital records since calculations involve the differences of large numbers. This accuracy, when combined with the requirements of a large dynamic signal level, results in a very sophisticated signal processing, and an increase in data handling by more than two orders of magnitude. This approach is expensive, and does not provide a filtered signal in real time for examination in the field.
Some of the existing geophysical systems use a very simplified but similar technique to provide filtering for coherent noise. The signal S+N (FIG. 1) is evaluated at known intervals during Ts. At a later time which is an integral multiple of 1/Fn and during Tn, the noise N is evaluated at the same known intervals. The readings of N are substracted from the corresponding readings of S+N to provide S. This provides results comparable to computer filtering but with reduced accuracy in the evaluation of N. This alleviates the volume of data handling but demands the same degree of incremental accuracy prior to evaluation of N and S+N.
One solution of the problem is to introduce a noise cancelling signal with the sensor at the input. Attempts have been made to utilize a second sensor to provide this signal. This sensor must reproduce the coherent noise with no signal S present. This is difficult since both S and N field patterns can have elliptical polarization. A balancing network is required for the noise sensor to adjust amplitude and phase of the noise and this restricts practical cancellation to a single frequency component. Cancellation can be obtained also from a single frequency artificial noise source. There is no knowledge of automation of these methods. These methods result in a relatively large time lapse between their adjustment and use of the cancelling signal. None of these methods is cost effective and have been used when results are essential in areas where approximate cancellation of noise justifies the results.
The standard procedure of sampling time-domain signals and noise reduction by "stacking" results is not suitable for the present problem. The effect of these procedures is dependent on incremental accuracy preceding their application.
A further object of the invention is to provide for noise cancellation at a low linear signal level and in real time.
Standard filtering is not satisfactory. The performance of conventional continuous wave (CW) filters is based on a continuous input. A series of CW band rejection or notch filters for Fn would be required. If time-domain signals such as St, S, were fed to such a CW filter combination, the continuous coherent noise would be removed but it would be impossible to recover the signal S.
The signal St can be 1000 times the desired signal S. The output due to St would have amplitude of the order of S and would "ring" (decaying oscillations) at the rejection frequencies. This "ringing" would extend St into the time band of S. The noise effect of this time distortion of St could be greater than the coherent noise which is rejected. The conventional CW filter has no storage capability and cannot be gated.
It is a particular object of the invention to provide method and apparatus for cancelling the coherent noise signal N with an identical signal N' before any nonlinear amplification is performed, with the cancelling signal N' having the character of the coherent noise signal during the period Tn and being independent of St and S. This may be achieved by storing the coherent noise signal during the period Tn for continuous reproduction to provide the desired cancelling signal N'. A particular object of the invention is to provide such a system utilizing an integrating type digital filter with digital memories in a feedback circuit of an operational amplifier. A further object is to provide such a system in an alternative embodiment incorporating condenser memory. An additional object of the invention is to provide such a system incorporating a means for advancing timing in the feedback circuit to compensate for a time delay in the system.
Other objects, advantages, features and results will more fully appear in the course of the following description.